The present invention relates generally to the field of electrostatic flow sorters, and more particularly to those sorters which are adapted to sense the presence and/or character of particles in a laminar flow stream and to selectively sort those particles by breaking that stream into a number of discrete droplets, and sorting those droplets containing such preselected particles. Such sorters are known for use in sorting and analyzing cellular compositions of given biological samples, as for example in the counting/analysis of cell types for a given blood sample.
In an apparatus of this general type, laminar flow is established through an area at which a light scattering, florescence or volume measurement is taken. Once a cell of interest has been sensed, an electronic time delay is normally activated for the length of time required for the cell to cover the distance from the point of cell detection to the point of droplet formation. Droplet formation may be accomplished by vibrating a flow chamber or orifice through which the stream passes, at a frequency sufficient to cause droplet formation, usually on the order of about 40,000 cycles per second. When a cell of interest arrives at the droplet formation point, a charging pulse may be applied to charge the droplet (plus, minus, or neutral) so that as the droplet of interest enters a subsequent DC field, it may be deflected as desired for collection. A general overview of this technique is provided in "Laser Flow Microphotometry For Rapid Analysis And Sorting Of Mammalian Cells", Mullaney, et al., Annals New York Academy Of Sciences, Vol. 267, pages 176-190 (see in particular, pages 180 and FIGS. 3 and 4).
Such particle sorters are also disclosed in U.S. Pat. Nos. 3,710,933 (Fulwyler, et al.) and 3,380,584 (Fulwyler) and 4,148,718 (Fulwyler). In these patents, sorting is accomplished in accordance with a selected parameter which may be size, volume, presence or radioactivity, color, fluorescence, light absorption or any quality capable of being translated into an electrical quantity. These patents additionally disclose single or multi parameter measurements to effect such sorting.
In order to selectively sort those droplets containing cells which are determined to be of particular interest, apparatus of this general type generally depends upon a flow rate estimate for the fluid containing a particular cell. This flow rate estimate is used to estimate the time between cell detection and the droplet breakpoint, at which selective charging of the droplet to be sorted takes place. As disclosed in U.S. Pat. No. 3,710,933, such systems are normally aligned and adjusted prior to taking cell measurements. In particular, droplet formation is normally checked by illuminating the emerging liquid jet near the flow chamber with a strobe light or equivalent light source. The strobe light is synchroflashed with respect to the oscillator frequency. Droplet formation can then be viewed using a microscope, and by varying the voltage and frequency applied to the stream perturbing transducer, droplet formation can be adjusted for a given nozzle diameter and flow rate. See U.S. Pat. No. 3,710,933, Column 11, lines 14-49.
As described particularly in U.S. Pat. No. 3,710,933, (Fulwyler, et al.), by pressurizing various reservoirs with known pressures, flow rates can be estimated and cell flow rate adjusted by varying the relative pressures between the various reservoirs feeding into the flow stream. The approximate time delay between cell sensing and droplet formation (which is estimated in Fulwyler, et al. to be in the order of 1400 microseconds) can be estimated so that an appropriate droplet charging generator will operate in combination with a pulse height analyzer and cell separation logic to charge the selected cell containing droplets for subsequent electrostatic sorting.
A number of factors affect the ability of a given apparatus to selectively sort one or more types of target cells from a continuous cell stream. Even assuming that the detection equipment for identifying each cell to be sorted is 100% accurate, differences in flow rate, temperature, fluid viscosity, and transducer performance can affect the time delay or location of the desired target-cell-containing droplets at the breakpoint, which is the point at which a charge pulse must be administered to insure that the target cell will be subsequently electrostatically sorted.
Heretofore, one of the methods used to adjust such a sorting apparatus involves running a test sample through that apparatus which is set or programmed to sort for one or more readily identifiable cell types. According to this procedure, the delay time is manually adjusted until those droplets which are sorted from the flow stream are found to contain the expected number of target cells. While this method, used alone or in combination with the stroboscopic method discussed above, has achieved some success in this art, it is prone to a certain degree of error, particularly during periods of extended machine use and/or changing operating conditions, such as changing sample viscosities and/or temperatures.
In U.S. Pat. No. 3,826,364 (Bonner, et al.), a particle sorting method and apparatus are disclosed wherein a coaxial flow stream is released through a vibrating nozzle. Inspection (interrogation) of the stream by one or more cell sensing means for sensing cells in the stream occurs immediately downstream of the nozzle. In the Bonner, et al device, charging pulses are supplied at appropriate times for proper separation of the drops through the use of delay units which are adjusted to provide the necessary time delay to allow for travel time of the particle from the point of particle scatter detection to the point where the stream breaks into drops. Bonner states:
"With the present arrangement the delay time between observation of a particle and its capture by a separating droplet is predictable to within three drop periods. Such high degree of predictability is due primarily to the uniform velocity of the inner particle containing stream 12A of the coaxial flow jet. That is, across the inner stream 12A the stream velocity is substantially uniform whereby particles anywhere within the cross-section of the inner stream travel with the same velocity from the point of observation to the drop separation point of the stream." U.S. Pat. No. 3,826,364, Col. 7, lines 22-32. PA1 "Obviously, if instrument tolerances, variations, drift and like permitted, then a drop charging time sufficient to charge only two successive drops, or a single drop, could be employed." U.S. Pat. No. 3,826,364, Col. 8, lines 2-6. PA1 "Selective drop charging involves the induction of charges in the drop being formed by a surrounding charged electrode. The induced charge varies in accordance with the inducing voltage until the instant in time when the droplet physically separates from the stream. From that time on, the induced charged is trapped and remains with the drop. It is obvious, therefore, that the charging process must be carefully synchronized with the timing of the drop break off. This involves the use of complex phasing control sensors and loops. This in turn, increases the cost of the equipment. . . . It is an object of this invention to provide an ink drop charging system which does not depend upon the synchronization of the charging with the break off time. PA1 "The ideal time to sense the frequency, phase, and amplitude components of the ink jet stream for determining drop break off characteristics is at the precise time droplets are formed therefrom. This is usually impossible to achieve, however, since the droplets are normally formed inside the charged electrode. Therefore, according to the present invention, the drop break off characteristics are determined by sensing upstream of break off, rather than downstream as taught by the prior art. The continuous portion, that is, the portion just prior to break off of the stream is sensed to determine the break off characteristics. In response to the sensed characteristics, a piezoelectric drive signal is provided which controls droplet formation, and accordingly provides increased drop charging efficiency." U.S. Pat. No. 4,047,143 (Taub), Col. 4, lines 53-68. PA1 "The light passing through the slit 44 is influenced by the wave passing a given point on the perimeter of the jet, and accordingly is a representation of the frequency components of the jet at this particular point, as well as being indicative of the shape of a given droplet when it breaks-off downstream. It is necessary to make this slit somewhat larger than the largest diameter to be measured, typically the drop diameter, so that the clipping of the wave form does not occur, as well as preventing the generation of spurious diffraction effects. A narrow band pass filter 48, which has a band pass on the order of 100 A centered in the laser wavelength, is used so measurements may be made in room light. The light passed by the filter 48 is then transmitted to a photomultiplier tube 50 which measures the intensity of the light. Therefore, the output voltage of the photomultiplier tube 50 is proportional to the diameter of the jet blocking the slit, which is to say, to the local diameter of the jet at the point being probed . . . . It is to be appreciated that the signal output . . . may be applied to analyzing means 80 by other timing means such as a stepping motor, or alternatively may be applied concurrently to inputs of devices 82, 84 and 86, rather than in the time sequence described." U.S. Pat. No. 4,047,183. See Col. 6, lines 27-68, Col. 7, lines 1-26.
As further explained in the Bonner, et al. disclosure, the duration as well as the time of application of the charging pulse is critical to the separation of at least the droplet containing the target particle to be sorted. After describing a preferred charging pulse which will charge at least three drops, Bonner, et al. states:
As also pointed out by Bonner, et al., a drop breaking from a given flow stream carries with it a charge which is proportional to the potential between the droplet stream and the surrounding electrodes or charging surfaces at the time the drop separates from the stream. If the drop breaks off from the jet stream during the transition time from the drop charge pulse, either during the leading or trailing edge of that pulse, some intermediate value between zero and the desired full charge may be imparted to the target droplet. In the Bonner, et al. apparatus, on/off transitions of the drop charging pulse are synchronized with the drop formation means, whereby charge pulse transitions may be synchronized to occur only intermediate the formation of droplets and not when droplets separate from the stream. This is made possible in the Bonner device by the provision of a variable phase control unit included in the transducer drive circuit which is adjusted for proper timing of droplet formation with the droplet charge pulse. As with the Fulwyler devices discussed above, stroboscopic illumination of the stream permits stream viewing through a suitable microscope, the stroboscopic illumination being synchronized by the drop charging pulses such that the stream, and more particularly the defleted drops, may be illuminated to ensure that the deflected drops contain the desired particles to be sorted.
More recently, various apparatus and method have been proposed for timing the application of a charge pulse so that droplets containing the particles to be sorted may be selectively charged. In U.S. Pat. No. 3,963,606 (Hogg), a particle separator is disclosed for separating particles in a fluid according to certain particle characteristics. The Hogg device includes a means for adjusting an electrical delay to be equal to the time between the emergence of a particle from a jet forming aperture to the point of break off. Hogg proposes the use of a movable scale in place of the ground glass of prior art projection microscopes, this scale being linked to a potentiometer of an RC oscillator to thereby control the oscillator's frequency. A second potentiometer for controlling the clock oscillator frequency is coupled to a height adjustment member of the aperture, this frequency being used to clock delay shift registers such that the charging pulse may easily be made to occur at the appropriate time, irrespective of fluctuations of pressure, velocity, amplitude and frequency of the droplet forming generator. See U.S. Pat. No. 3,963,606, (Hogg) Col. 2, lines 23-36. Accordingly, Hogg represents a more automated version of the stroboscopic projection microscopic techniques discussed above.
Droplet forming characteristics in a perturbed stream have also been considered in connection with the art of ink jet printing. In the ink jet printing art, where discrete ink droplets formed in an ink jet stream are electrostatically directed to form characters on a recording surface, particular attention has been paid to establish uniform droplet formation and charging characteristics. Since the charge imparted to any given droplet at its breakpoint is proportional to its surface area, i.e., the shape of that droplet at the breakpoint, and since even slight charge variations may produce erratic deflection characteristics, ink jet printing artisans have proposed various systems for producing an ink jet stream comprising uniformly shaped and uniformly charged droplets which will exhibit predictable down stream deflection behavior. These problems are complicated by the tendency of perturbed streams to form "satellites" which not only affect the charge imparted to preceeding or succeeding droplets, but also alter the volume of those droplets, thereby correspondingly affecting print uniformity.
In the ink jet printing art, numerous systems have been proposed for sensing the characteristics of a perturbed ink jet stream, either above or below the breakpoint of that stream. U.S. Pat. No. 3,907,429 (Kuhn, et al.) discloses a method and device for detecting the velocity of droplets formed from a liquid stream. According to this disclosure, discrete droplets are directed between a pair of apertures and a light source which is strobed at a selected frequency and directed towards the apertures. By detecting the time between when a first of the apertures is blocked by a droplet in the stream as indicated by the light being broken during the strobe and the time when a second of the apertures is blocked by another droplet, when the light source is counted, the velocity of the droplets may be measured and a correction of the velocity made by changing the pressure of the manifolds supplying the liquid stream. In U.S. Pat. No. 3,769,627 (Stone) an ink jet printing system using ion charging of droplets is disclosed wherein a light source and photocell located downstream from the breakpoint of a perturbed stream is used to sense the passage of discrete droplets and to time delayed charges subsequently applied thereto. Stone states:
It is another object of this invention to produce an ink drop charging system, which charges drops after they break off from the ink jet stream." U.S. Pat. No. 3,769,627 (Stone), Col. 1, lines 18-35.
This method is accomplished by using the above-described photocell arrangement for the purpose of counting and synchronizing charges applied as discrete droplets pass a plurality of separate charging stations which respond to coded information applied to each station in synchronism with the passage of each drop.
As disclosed in U.S. Pat. No. 4,047,183 (Taub), efforts have also been made to control the formation and shape of droplets in an ink jet stream by sensing the surface wave profile of the continuous portion of the stream (upstream from the breakpoint) by illuminating that portion of the stream with a radiant energy source such as a laser. The surface wave profile produced by illuminating the stream is sensed to provide the fundamental and harmonic frequency components thereof, and a perturbation drive signal, the amplitude and relative phase of which is a function of the sensed frequency components, is provided for controlling the formation and shape of the droplets. After discussing the advantages and difficulties of controlling the break off geometry, particularly with the respect to the illumination of satellite formations, Taub discloses the practical desirability of measuring the ink jet stream upstream rather than downstream from the droplet break off point:
Taub discloses a system wherein an ink jet manifold having a perturbation means such as a piezoelectric crystal emits a perturbed ink jet stream into charge electrode structures which are pulsed in "a well known manner" to selectively apply charge to the droplets. A source of radiant energy, which may comprise a helium-neon laser, emits radiant energy focused on the continuous portion of the jet "just prior to the jet entering the charged electrode structure". "Since the ink is opaque, a shadow is formed" which is imaged through a lens onto a substrate which has a slit formed therein. The shadow formed thereby represents the surface wave profile of the jet which is a representation of the respective amplitudes and relative phases of fundamental and harmonic frequencies. Taub states:
In Taub's preferred embodiment, the output signal so obtained is conditioned to control the fundamental and harmonic frequencies applied to the piezoelectric perturbation means for controlling the droplet formation and shape of droplets produced by the ink jet stream.
For other disclosures of ink jet printing systems using optical sensors, see IBM Technical Disclosure Bulletin Volume 16, No. 12, May 1974, Page 3877-8, entitled "Feedback For Synchronized Pressure Jet Using Optical Sensor", and IBM Technical Disclosure Bulletin, Vol. 16, No. 3, August 1973, Page 880, entitled "Phase Detection On Ink Jet Droplets".
For other disclosures relating to various ink jet printing synchronization systems, please refer to U.S. Pat. No. 4,025,926 (Fujimoto, et al.) entitled, "Phase Synchronization For Ink Jet System Printer"; U.S. Pat. No. 4,045,770 (Arnold, et al.) entitled, "Method and Apparatus For Adjusting The Velocity Of Ink Drops In An Ink Jet Printer"; U.S. Pat. No. 3,953,860 (Fujimoto, et al.) entitled, "Charge Amplitude Detection For Ink Jet System Printer"; U.S. Pat. No. 3,761,941 (Robertson) entitled, "Phase Control For A Drop Generating and Charging System"; U.S. Pat. No. 3,836,912 (Ghougasian, et al.) entitled, "Drop Charge Sensing Apparatus For Ink Jet Printing System"; U.S. Pat. No. 3,982,251 (Hochberg) entitled, "Method and Apparatus For Recording Information On a Recording Medium"; U.S. Pat. No. 3,878,519 (Eaton) entitled, "Method and Apparatus For Synchronizing Droplet Formation In A Liquid Stream".
For other patents disclosing particle or flow sorting systems, please see U.S. Pat. No. 3,941,479 (Whitehead) entitled, "Use Of Modulated Stimulus To Improve Detection Sensitivity For Signals From Particles In A Flow Chamber"; U.S. Pat. No. 3,851,169 (Faxvog) entitled, "Apparatus For Measuring Aerosol Particles"; and U.S. Pat. No. 3,910,702 (Corll) entitled, "Apparatus For Detecting Particles Employing Apertured Light Emitting Device".